Wireless automatic meter reading systems are well known. Typically, each utility meter is provided with a battery-powered encoder that collects meter readings and periodically transmits those readings over a wireless network to a central station. The power limitations imposed by the need for the encoder to be battery powered and by regulations governing radio transmissions effectively prevent direct radio transmissions to the central station. Instead, wireless meter reading systems typically utilize a layered network of overlapping intermediate receiving stations that receive transmissions from a group of meter encoders and forward those messages on to the next higher layer in the network as described, for example, in U.S. Pat. No. 5,056,107. These types of layered wireless transmission networks allow for the use of lower power, unlicensed wireless transmitters in the thousands of end point encoder transmitters that must be deployed as part of a utility meter reading system for a large metropolitan area. In a related mode, the remote meter encoders are read via handheld reader/programmers vans that contain RF reading equipment.
Since a number of these devices operate on batteries, preserving battery life is one of the main concerns. By way of example, a battery operated RF encoder used to read gas utility meters typically goes to “sleep” between reads in order to conserve battery power. When a van passes through the area, it sends out a “wake-up” signal that causes the Encoder/Receiver/Transmitters (i.e., the endpoints) in the area to respond by transmitting encoded signals containing the metering data and any stored tamper signals. While at short distances it is possible to establish two-way RF communications with the automatic meter reading (AMR) system using a van or handheld devices, it would be more efficient to configure these fixed AMR systems into full two-way RF communication systems that remotely transmit the data to the central office of a utility company (such as through a network of concentrators located throughout the AMR system).
Complicating factors to reaching a goal of full two-way communication include: the great number and variety of devices that exist in the field (gas, water, electric) that have to communicate back to the central office; the different utilities that have to communicate with their respective devices within the different geographical locations and the types of data being requested; and interference and collisions caused by several reading devices communicating their data back to the central office. The increase in density of devices in relation to the geographic area and the need for simultaneous communication begin to approximate the issues already being faced by telecommunication companies with their cellular communication networks. Similarly, some fixed AMR systems are being geographically configured to simulate cellular communication systems in that they are comprised of different cells that transmit their data and communicate primarily over a single frequency channel.
Due to the number of transactions per cell, which is comparable to the number of utility meters in an AMR system, and due to the time it takes for a request/answer transaction to be processed, each concentrator will keep its RF frequency (“channel”) busy for a period of time during which no other relatively close cells can use the same RF channel (thereby avoiding co-channel interference or collisions). Thus, when using the fixed network schemes of the prior art on a single channel, there are limitations as to the number of cells which could be read on any given day. Therefore, in certain geographical areas it is difficult to meet the requirements of simply performing the basic AMR operation in a reasonable amount of time.
One approach is to assign different frequencies to the different devices or to the different cells in the AMR system. However, this is only a short-term solution as the number of available frequency bands is inadequate to handle the increasing number of endpoint units that will be placed in a defined geographic location. Increasing the number of endpoints in the AMR system requires more efficient utilization of the limited available frequency spectrum in order to provide more total channels while maintaining communications quality. This is even more of a challenge where the endpoints are not uniformly distributed among cells in the system. More channels may be needed for particular cells to handle potentially higher local endpoint densities at any given time. For example, a cell in an urban area will include more endpoints requiring reading then a cell in a rural area.
For many of these same reasons, conventional cellular systems utilize frequency reuse to increase potential channel capacity in each cell and increase spectral efficiency. Frequency reuse involves allocating a frequency to each cell, with cells utilizing the same frequencies geographically separated to allow cellular phones in different cells to simultaneously use the same frequency without interfering with each other. By so doing, many thousands of subscribers may be served by a system providing only several hundred frequencies. One of the drawbacks to using frequency or cellular reuse in AMR systems is the limited number of available frequency bands.
Accordingly, there is a need for a system for collecting data from meter modules located in a wide area that has a high degree of accuracy and reliability without substantially increasing the costs of the AMR network. An approach that addresses the aforementioned problems, as well as other related problems, is therefore desirable.